This invention concerns metal working and more specifically the shaping of a workpiece by removing material by electrical erosion.
It is well-known that cutting tools have been relieved to provide clearance with the profile of the tool being maintained constant in planes parallel to the cutting face so that, as the tool was repeatedly sharpened, the cutting edge would retain the same shape.
When shapes were produced by electrical erosion of electrically conducting plates, the plate was positioned horizontally and a wire electrode was positioned vertically while the plate and electrode were moved relative to each other in a horizontal plane as an electric current was passed between them to erode the workpiece in such a manner as to produce the desired shape. The electric current removed material from the workpiece throughout a working gap, which required that the electrode be spaced from the periphery of the desired shape by a distance equal to the radial length of the working gap, which, under these conditions, would be uniform in all radial directions. This made programming of the relative motion very simple.
If the erosion was produced by electrical discharge machining, a stream of dielectric fluid was directed into the working gap to carry away the eroded material and so prevent a short-circuit, which would burn the electrode and the workpiece. If the erosion was produced by electrolysis, a stream of electrolyte was required to carry the electrical current between the electrode and the workpiece, thereby permitting erosion to occur and carrying away the eroded material. This proved to be very satisfactory for producing unrelieved shapes.
For producing relieved shapes, the workpiece was tilted at the desired relief angle from horizontal and the electrode and workpiece were moved relatively in a horizontal plane. It was a simple matter to project the desired shape on the horizontal plane and to maintain the electrode spaced from the periphery of the projected shape by the predetermined fixed radial length of the working gap. Trouble was encountered, however, by the stream of fluid tending to flow down the sloping surface of the workpiece rather than into the working gap. As a result, short-circuits were created between the electrode and the workpiece when the eroded material was not removed during electrical discharge machining and material erosion ceased or was reduced when electrolyte was diverted from the working gap during electrolysis. Another problem involved the accuracy of the produced shape. Because the electrode is guided above and below the workpiece, the spacing between the guides had to be greater in order to clear the workpiece at its uppermost point and at its lowermost point on the desired shape. The longer unsupported length of electrode between the guides permitted more deviation from its desired path with relation to the profile of the desired shape. This is more easily recognized when it is understood that the electrode vibrates.
In an attempt to cure these problems, the work-piece was maintained in horizontal position while the electrode was tilted at the desired relief angle from vertical. The fluid was thus prevented flow flowing away from the working gap and the spacing between electrode supports could be reduced, since they were in a fixed vertical relation to the workpiece. A problem arose in maintaining the electrode at the proper distance from the periphery of the desired shape, which problem was evidenced by inaccuracies in the produced shapes.